Hot-top continuous casting may be seen as a development of conventional continuous casting, which is manifested by the fact that the meniscus (free surface of the cast metal) is pushed back upwards with respect to the level where the solidification of the metal against the cooled copper of the mould is initiated, whereas these two levels are virtually coincident in conventional continuous casting. This novel arrangement is obtained by placing, on the cooled copper part of the mould, a contiguous feed head made of thermally insulating refractory material, intended to contain the cast liquid metal and on the wall of which feed head any corresponding spurious solidification must be avoided. Consequently, the solidification of the cast metal can start correctly on the upper edge of this copper part. For this reason an inert gas (argon, for example) is injected along the first inner perimeter of the mould between the copper part and the feed head, in the form of jets intended to shear any undesirable solidified film which would have a tendency to be already formed in contact with the refractory feed head. This type of arrangement is described in document FR-A-2,703,609, the content of which is incorporated into the present specification by way of reference.
As taught by Patent Application FR-A-2,747,061, it is therefore advantageous for the bottom of the feed head to consist of a part made of a dense refractory having good mechanical strength, such as Sialon.RTM.. This part acts as a transition region between the cooled copper of the mould and the thermally insulating fibrous refractory of the feed head placed above it, which would degrade too rapidly if it were placed directly in contact with the upper edge of the cooled copper body, at the point where definite solidification of the cast metal starts. On the other hand, the risk of spurious premature solidification on this intermediate part is increased, but remains of no consequence because of the shearing gas blown into the interface with the copper, which interrupts the downward propagation of this undesirable process.
The stream of shearing gas is injected via a thin slot (a few tenths of a millimetre barely suffice), which is produced by compressing a bead of fibrous refractory material placed between the Sialon insert and the copper body of the mould. Using clamping means, the bead is compressed until the desired slot thickness is obtained, this being calibrated using controlled shims.
However, it proves to be necessary for correct execution of the casting process for there to be uniform distribution of the flow of gas around the inner perimeter of the mould. Now, despite every care that may be taken in controlling the slot thickness "when cold" (in the absence of cast metal), this good linear distribution is not in general correctly provided. Moreover, account cannot be taken of local disparities in the pressure drop at the copper/refractory interface, which disparities are associated, inter alia, with local variations in the microroughness of the two facing surfaces which define the injection slot. Furthermore, there is even less uniformity when operating "hot" (in the presence of cast metal) because of the phenomena of differential expansion of the materials involved.